Such a method is already known from United States patent application publication 2011/0004437 and is used in order to avoid so-called erroneous contacts. Before the particular solution of United States patent application publication 2011/0004437 is examined, the background to such erroneous contacts is firstly described. Thus, in the past such coordinate measuring devices usually had only one probe sensor in the form, for example, of a piezoelectric sensor or a strain gauge whose probe signal was evaluated in order to determine the contact time point, and thus to determine the associated scale values. Here, it is possible during measurement operation that the probe unit is so strongly accelerated by accelerations of the probe head when contact has been made with a work piece that the probe sensor thereby generates a probe signal on the basis of which a contact is assumed. Accelerations of the probe head by which such erroneous contacts can be initiated can, for example, be caused by vibrations of the coordinate measuring device, by accelerations of the linear axis drives of the coordinate measuring device, by accelerations of the rotary axis drives of a rotary swivel joint, by structureborne sound or by airborne sound. Even poor linear axis drives of the coordinate measuring device, which cause rough running, can already lead to such erroneous contacts.
In order to suppress such erroneous contacts, there have already been provided in the past different trigger conditions that generally must be cumulatively satisfied so that valid contact occurs. Aside from the overshooting of a trigger threshold by the probe signal, by way of example a supplementary trigger condition has been added in the form of a check as to whether the probe signal overshoots the trigger threshold for a predetermined duration. However, this can likewise lead to erroneous contacts in the case of comparatively long-lasting accelerations.
Our U.S. Pat. No. 5,862,604, U.S. Pat. No. 5,526,576 and U.S. Pat. No. 5,425,180 show evaluation circuits in the case of which it is not only the probe signal that is used to trigger a contact in order to initiate a contact. Rather, the probe unit is additionally also further deflectably supported on the probe head, a switch element being provided that outputs a switching signal upon a deflection. What is required here as trigger conditions for a valid contact is that the probe signal exceeds a trigger threshold, that the switching signal is output and that the overshooting of the trigger threshold and the outputting of the switching signal be performed within a predetermined time period. Despite these trigger conditions, erroneous contacts can also occur here given accelerations of the probe head. This is the case when because of accelerations of the probe head the probe signal overshoots the trigger threshold and a contact does take place shortly thereafter and the switching signal is also thereby output. Since the scale values are read out on the basis of the probe signal, the contact time point determined herefrom then lies partly clearly ahead of the actual contact time point, and this leads to defective measured values.
In order to avoid such erroneous contacts caused by acceleration, United States patent application publication 2011/0004437 proposes a method in which, aside from the probe sensor that outputs an appropriate probe signal given contact of a work piece with the probe unit, there is provided in the probe head an acceleration sensor that supplies an acceleration signal upon accelerations of the probe head. In order to avoid erroneous contacts, for the case when the acceleration sensor supplies an acceleration signal, a change is made to the trigger conditions on the basis of which the scale values which represent the position of the probe head are adhered to. In particular, the level of a predefined threshold value that, must be overshot by the probe signal is raised, and the duration in the course of which the threshold value thereof must be overshot by the probe signal is extended by a predefined time. As a result of the increase of the threshold value to be overshot, and of the extension of the predefined time in the course of which the threshold value must be overshot before acceptance of the scale values is initiated, the probe head is “desensitized”, and the contact time point defined by the trigger is therefore ever further removed from the actual contact time point. Further, because of this the conditions under which a contact is detected change continuously as a function of the acceleration. The contacting thereby becomes imprecise.